Reductive

Article pubs.acs.org/joc

A Palladium-Catalyzed Ullmann Cross-Coupling/Reductive Cyclization Route to the Carbazole Natural Products 3‑Methyl‑9H‑carbazole, Glycoborine, Glycozoline, Clauszoline K, Mukonine, and Karapinchamine A Qiao Yan, Emma Gin, Malgorzata Wasinska-Kalwa, Martin G. Banwell,* and Paul D. Carr Research School of Chemistry, Institute of Advanced Studies, The Australian National University, Canberra, Australian Capital Territory 2601, Australia S Supporting Information *

ABSTRACT: The title natural products 2−7 have been prepared by reductive cyclization of the relevant 2-arylcyclohex-2-en-1-one (e.g. 20) to the corresponding tetrahydrocarbazole and dehydrogenation (aromatization) of this to give the target carbazole (e.g. 4). Compounds such as 20 were prepared using a palladium-catalyzed Ullmann cross-coupling reaction between the appropriate 2iodocyclohex-2-en-1-one and o-halonitrobenzene.

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Sometime ago we reported10 that 1,2,3,4-tetrahydro-9Hcarbazoles (formally 2,3,4,9-tetrahydro-1H-carbazoles) can be formed by a two-step process involving an initial the palladiumcatalyzed Ullmann cross-coupling of 2-halocyclohex-2-en-1ones with o-halonitrobenzenes and then subjecting the resulting 2-arylcyclohex-2-en-1-ones to a reductive cyclization reaction.11 Since various methods are available for or could be applied to the oxidation of tetrahydrocarbazoles to carbazoles,12 we sought to establish if the reaction sequence just mentioned could provide a useful means for obtaining natural products embodying the latter ring system. Herein we report the outcomes of such studies and by which means we have been able to realize syntheses of the parent carbazole (1) as well as the natural products 3-methyl-9H-carbazole (2),13 glycoborine (3, aka glycrophylamine),3a,14 glycozoline (4),3a,15 clauszoline K (5),16 mukonine (6),17 and karapinchamine A (7)18 together with their monomethoxylated congener 8 (Figure 2).

INTRODUCTION 9H-Carbazole (1) (Figure 1) was first isolated from coal tar more than 100 years ago,1 and since that time this aromatic

Figure 1. Structure of parent 9H-carbazole (1) and the associated ring labeling and atom numbering.

heterocycle and its various derivatives have fascinated organic chemists because of their value in both medicine and materials science.2 Many biologically active natural products embodying this framework have also been isolated, particularly from higher plants.2d,e,3 As such, the development of methods for the synthesis of the carbazoles has been an ongoing field of research. A suite of approaches to these compounds has been reported, ranging from the classical Fischer−Borsche4 and Graebe−Ullmann5 routes to more contemporary ones such as the cyclization of biarylnitrenes (Cadogan synthesis)6 or variants thereof and the annulation of indoles,7 including through electrocyclization processes. Despite the demonstrated utility of these and other approaches,8 perhaps the most effective route to carbazoles involves the cyclization of diarylamines, especially under oxidative conditions.9 Variations on this last approach have been used to great effect in developing total syntheses of a plethora of carbazole-containing natural products, with particularly notable contributions having being made, especially in recent times, by the Knölker group.2,9e−i,k,m © 2017 American Chemical Society

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RESULTS AND DISCUSSION Our initial studies focused on acquiring targets 1 and 2, and this involved (Scheme 1) the initial palladium-catalyzed Ullmann cross-coupling of 2-iodocyclohex-2-en-1-one (9)19 or its C4methylated counterpart 1011 with commercially available obromonitrobenzene (11) under conditions defined earlier,10 and thereby affording the anticipated and previously reported 2-arylcyclohex-2-en-1-ones 1210,11 (85%) and 1311 (86%), respectively. A methanolic solution of each of compounds 12 and 13 was then subjected to reaction with hydrogen in the presence of commercially available W-2 Raney nickel at room temperature, and so affording the tetrahydrocarbazoles 1410,11,12b (64%) and 1511,12b (58%), respectively. Various Received: January 8, 2017 Published: February 22, 2017 4148

DOI: 10.1021/acs.joc.7b00044 J. Org. Chem. 2017, 82, 4148−4159

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The Journal of Organic Chemistry

Figure 2. Structures of natural products 2-7 and the monosubstituted congener 8.

Scheme 1. Reaction Sequences Leading to Carbazoles 1 and 2

Scheme 2. Synthesis of Glycoborine (3)

secured by a single-crystal X-ray analysis.20 The appearance of 28 signals in the 13C NMR spectrum of this C14 compound suggested that it existed as diastereoisomeric atropisomers under ambient conditions. Reductive cyclization of compound 17 using hydrogen in the presence of W-2 Raney nickel gave the tetrahydrocarbazole 1811a (72%), and dehydrogenation of this compound under the same conditions as employed before then afforded the target natural product 3, that was obtained in 75% yield. Once again, all of the appropriate spectral comparisons left no doubt that the glycoborine had been obtained but final confirmation of this followed from a singlecrystal X-ray analysis.20 A reaction sequence essentially analogous to that described above was used to synthesize glycozoline (4), a compound that has also been the target of previous studies.3c,4c,6i,12b,22 Thus, as shown in Scheme 3, reaction of 2-iodo-4-methylcyclohex-2-en1-one (10) with commercially available nitroarene 19 under the by now standard palladium-catalyzed Ullmann cross-coupling conditions gave the anticipated product 20 (87%), which was reductively cyclized with hydrogen in the presence of W-2

attempts to modify these reductive cyclization conditions in an effort to obtain dihydrocarbazoles (or perhaps even the carbazoles themselves as a result of conducting the workup under aerobic conditions) were unsuccessful. In the final step of the reaction sequence, then, mesitylene solutions of compounds 14 and 15 were each heated to 150 °C with an equal weight of 10% palladium on carbon, and thus affording carbazoles 1 (55%) and 212b (57%), respectively. All of the spectroscopic and physical data acquired on these reaction products were in complete accord with the assigned structures and matched those derived from a commercially available sample (in the former case) or reported6h,12b in the literature (in the latter case). Furthermore, each was subjected to singlecrystal X-ray analysis.20 The preparation of glycoborine (3), a synthetic target pursued by others,6e,g,k,9h,k,14 was readily accomplished as shown in Scheme 2 by coupling 2-iodo-4-methylcyclohex-2-en1-one (10)11 with nitroarene 1621 and thereby producing the arylated cyclohexenone 17 (88%), the structure of which was 4149


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The Journal of Organic Chemistry

(80%), and this in turn engaged in the same type of reductive cyclization reaction as seen before to give tetrahydrocarbazole 2411a,14 (65%). However, compound 24 proved rather prone to oxidation, with the hydroperoxide 25 being formed in increasing quantities when its precursor was allow to stand at 22 °C as a solution (in various solvents) left open to the atmosphere. Compound 25, the structure of which was established by single-crystal X-ray analysis,20 may arise through a facially selective ene reaction between indole 24 and singlet oxygen, the latter reactant (most likely) being produced through the other (24) serving as a sensitizer. Despite the ease of the conversion 24 → 25, the tetrahydrocarbazole was readily dehydrogenated in the same manner as described in the other instances reported herein, producing carbazole 269c,h,22d,23a,b in 88% yield. Unlike isomer 4, compound 26 is not a naturally occurring material but is readily converted into one, namely aldehyde 5 (clauszoline K) (66%), on treatment with DDQ in aqueous methanol9h,23a,b at ambient temperature for 4 h. The structure of compound 5 was confirmed by singlecrystal X-ray analysis.20 The carbazole natural product mukonine (6), another popular target compound,9a,12d,17a,24 carries both the methoxy and carbomethoxy substituents in the same ring and so requiring, as the first step in the present synthesis, the palladium-catalyzed Ullmann cross-coupling of the “parent” 2iodocyclohexeneone 9 with the readily obtained (see below) tetrasubstituted arene 27 (Scheme 5), affording the required 2arylated cyclohexenone 28, albeit in just 39% yield. Reductive cyclization of compound 28 under standard conditions gave tetrahydrocarbazole 29 (79%), which upon dehydrogenation under the usual conditions afforded mukonine (6) (66%) as a white, crystalline solid. Once again, all of the derived spectral

Scheme 3. Synthesis of Glycozoline (4)

Raney nickel and so providing the tetrahydrocarbazole 2112b,d,22b in 72% yield. Dehydrogenation of compound 21 through brief treatment with 10% Pd on C in diphenyl ether at 210 °C then gave glycozoline (4) in 89% yield, the structure of which was confirmed by single-crystal X-ray analysis.20 The reaction sequence leading to clauszoline K (5) (Scheme 4), another popular target,22d,23 provided some insights into the propensity of 2,3,4,9-tetrahydro-1H-carbazoles to engage in alternate oxidation reactions.12a Thus, the palladium-catalyzed Ullmann cross-coupling of the iodinated cyclohexenone 10 with the commercially available 2-iodonitroarene 22 proceeded as anticipated to give the required arylated cyclohexenone 23 Scheme 4. Synthesis of Clauszoline K (5)

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The Journal of Organic Chemistry Scheme 5. Synthesis of Mukonine (6)

Scheme 7. Synthesis of 2-Methoxy-9H-carbazole (8)

data were fully consistent with the assigned structure as well as those reported for the natural product, but final confirmation of this followed from a single-crystal X-ray analysis.20 The arene 27 used in this sequence was prepared by first brominating commercially available methyl 4-amino-3-methoxybenzoate with N-bromosuccinimide (NBS) and then oxidizing the previously reported25 bromide to the corresponding nitro compound (viz. 27) using m-chloroperbenzoic acid (mCPBA) (see the Experimental Section for details). The trisubstituted carbazole natural product karapinchamine A (7), the subject of just one previous synthetic study,7e was readily obtained (Scheme 6) by treating compound 26 with boron tribromide so as to effect cleavage of the methyl ether moiety and then treating the product hydroxycarbazole 309h (84%), as a solution in THF, with an excess of n-butyllithium and then ca. 1.5 equiv of geranyl bromide. By such means karapinchamine A (7) was obtained in 50% yield after column chromatography. Interestingly, there was no evidence for the formation of the isomeric O-geranylated product during the course of this reaction. All of the spectral data acquired on product 7 matched those recorded for the natural product (see the Supporting Information for relevant spectral comparisons). Monosubstituted carbazoles wherein the single substituent derives from the arene coupling partner are also readily available by the procedures reported here. Thus, as shown in Scheme 7, cross-coupling of compounds 9 and 22 under the usual conditions afforded enone 3110,11b (91%), which on subjection to reductive cyclization using hydrogen in the presence of Raney nickel afforded the tetrahydrocarazole 3210,11b,26 (76%). Dehydrogenation of compound 32 using 10% Pd on C in hot mesitylene then afforded carbazole 86f,k,l,9j

(64%), the spectral data for which matched those reported previously.

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CONCLUSIONS The procedures detailed here allow for the straightforward preparation, in a fully regiocontrolled manner, of a range of carbazoles carrying various combinations of substituents in both the A and the C rings as well as on the nitrogen of the B ring. Substitution patterns encountered in naturally occurring and biologically active carbazoles appear to be completely accessible using the methods described here. As a further indication of the utility of the title protocol, it is worth noting that carbazole 30 is an established precursor to the pyrano[3,2a]carbazole alkaloids isogirinimbine and mahanimbicine.9h In a related vein, clauszoline K (5) is an established synthetic precursor to the alkaloids clauszoline M and N (the corresponding methyl ester and acid, respectively).22d,23a The present study also serves to highlight the continued utility of the palladium-catalyzed Ullmann cross-coupling reaction,27 especially when it is used in combination with reductive cyclization protocols.28

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EXPERIMENTAL SECTION

General Experimental Procedures. Unless otherwise specified, proton (1H) and carbon (13C) NMR spectra were recorded at 18 °C in base-filtered CDCl3 on a spectrometer operating at 400 MHz for proton and 100 MHz for carbon nuclei. 1H NMR data are recorded as follows: chemical shift (δ) [multiplicity, coupling constant(s) J (Hz), relative integral], where multiplicity is defined as s = singlet, d = doublet, t = triplet, q = quartet, and m = multiplet, or combinations of the above. In relevant cases, the signal due to residual CHCl3 appearing at δH 7.26 and the central resonance of the CDCl3 “triplet”

Scheme 6. Synthesis of Karapinchamine A (7)

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The Journal of Organic Chemistry appearing at δC 77.0 were used to reference 1H and 13C NMR spectra, respectively. Samples were analyzed by infrared spectroscopy (νmax) as thin films on KBr plates. Low- and high-resolution electron impact (EI) mass spectra were recorded on a double-focusing, triple-sector machine. Low- and high-resolution ESI mass spectra were recorded on a triple-quadrupole mass spectrometer operating in either positive or negative ion mode. Melting points are uncorrected. Analytical thinlayer chromatography (TLC) was performed on aluminum-backed 0.2 mm thick silica gel 60 F254 plates. Eluted plates were visualized using a 254 nm UV lamp and/or by treatment with a suitable dip followed by heating. These dips included phosphomolybdic acid/ceric sulfate/ sulfuric acid (concentrated)/water (37.5 g/7.5 g/37.5 g/720 mL), potassium permanganate/potassium carbonate/5% sodium hydroxide aqueous solution/water (3 g/20 g/5 mL/300 mL), and p-anisaldehyde or vanillin/sulfuric acid (concentrated)/ethanol (15 g/2.5 mL/250 mL). Flash chromatographic separations were carried out following protocols defined by Still et al.29 with silica gel 60 (40−63 μm) as the stationary phase and using the AR- or HPLC-grade solvents indicated. The melting points of solids purified by such means were recorded directly (i.e. after they had crystallized from the concentrated chromatographic fractions). Starting materials, reagents, drying agents, and other inorganic salts were generally commercially available and were used as supplied. The copper powder used in the palladiumcatalyzed Ullmann cross-coupling reactions had a particle size of 2.0σ(I)]; Rw = 0.058 (all data), S = 1.00. Compound 2: C13H11N, Mr = 181.24, T = 150 K, monoclinic, space group P21/c, Z = 4, a = 20.3990(8) Å, b = 5.7846(2) Å, c = 7.8899(3) Å; β = 98.458(4)°; V = 920.88(6) Å3, Dx = 1.307 g cm−3, 1858 unique data (2θmax = 147.8°), R = 0.057 [for 1748 reflections with I > 2.0σ(I)]; Rw = 0.096 (all data), S = 1.00. Compound 3: C14H13NO, Mr = 211.26, T = 150 K, monoclinic, space group P21/n, Z = 4, a = 11.4714(17) Å, b = 5.6130(6) Å, c = 16.915(2) Å; β = 95.648(12)°; V = 1083.9(2) Å3, Dx = 1.295 g cm−3, 2160 unique data (2θmax = 151.8°), R = 0.058 [for 1763 reflections with I > 2.0σ(I)]; Rw = 0.164 (all data), S = 1.00. Compound 4: C14H13NO, Mr = 211.26, T = 150 K, orthorhombic, space group P212121, Z = 4, a = 6.4420(1) Å, b = 7.5921(1) Å, c = 21.9422(3) Å; V = 1073.16(3) Å3, Dx = 1.308 g cm−3, 2166 unique data (2θmax = 147.8°), R = 0.035 [for 2114 reflections with I > 2.0σ(I)]; Rw = 0.064 (all data), S = 1.00. Compound 5: C14H11NO2, Mr = 225.25, T = 150 K, orthorhombic, space group Pbca, Z = 8, a = 7.1367(1) Å, b = 13.2390(1) Å, c = 22.5108(2) Å; V = 2126.88(4) Å3, Dx = 1.407 g cm−3, 2152 unique data (2θmax = 147.8°), R = 0.033 [for 2089 reflections with I > 2.0σ(I)]; Rw = 0.058 (all data), S = 1.00. Compound 6: C15H13NO3, Mr = 255.27, T = 150 K, monoclinic, space group C2/c, Z = 8, a = 15.7767(2) Å, b = 9.4629(1) Å, c = 16.1402(2) Å; β = 91.7342(13)°; V = 2408.52(5) Å3, Dx = 1.408 g cm−3, 3284 unique data (2θmax = 59.6°), R = 0.039 [for 2869 reflections with I > 2.0σ(I)]; Rw = 0.077 (all data), S = 1.00. Compound 17: C14H15NO4, Mr = 261.27, T = 150 K, monoclinic, space group P21/c, Z = 8, a = 21.5986(4) Å, b = 6.89910(11) Å, c = 17.6362(3) Å; β = 104.596(2)°; V = 2543.16(8) Å3, Dx = 1.365 g cm−3, 5147 unique data (2θmax = 147.6°), R = 0.084 [for 4826 reflections with I > 2.0σ(I)]; Rw = 0.214 (all data), S = 1.12. Compound 25: C14H17NO3, Mr = 247.29, T = 150 K, monoclinic, space group P21/c, Z = 4, a = 14.6135(2) Å, b = 5.8504(1) Å, c = 14.6942 (2) Å; β = 96.2796(15)°; V = 1248.74(3) Å3, Dx = 1.315 g cm−3, 2542 unique data (2θmax = 147.6°), R = 0.036 [for 2433 reflections with I > 2.0σ(I)]; Rw = 0.098 (all data), S = 1.00. Structure Determination. The image for compound 6 was measured on a diffractometer (Mo Kα, graphite monochromator, λ = 0.71073 Å) fitted with an area detector, and the data were extracted using the DENZO/Scalepack package.34 Images for compounds 1−5, 17, and 25 were measured on a diffractometer (Cu Kα, mirror monochromator, λ = 1.54184 Å) fitted with an area detector and the data extracted using the CrysAlis package.35 The structure solutions for all eight compounds were solved by direct methods (SIR92)36 then refined using the CRYSTALS program package.37 Atomic coordinates, bond lengths and angles, and displacement parameters have been deposited at the Cambridge Crystallographic Data Centre (CCDC nos. 1525166, 1525163, 1525164, 1525162, 1525167, 1525160, 1525161, and 1525165 for compounds 1−6, 17, and 25, respectively). These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, by e-mailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax: + 44 1223 336033). Singlecrystal X-ray analyses of compound 1 have been reported previously.38

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b00044. Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data and anisotropic displacement ellipsoid plots derived from the single-crystal X-ray analyses of compounds 1−6, 17, and 25, 1H and 13C NMR spectra of compounds 1−15, 17, 18, 20, 21, 23− 32, and methyl 4-amino-3-bromo-5-methoxybenzoate (precursor to compound 27) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.G.B.: [email protected]. ORCID

Martin G. Banwell: 0000-0002-0582-475X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Australian Research Council and the Institute of Advanced Studies for financial support. Q.Y. is the grateful recipient of a PhD Scholarship provided by China Scholarship Council of the People’s Republic of China. E.G. acknowledges scholarship support from the ANU. M.W.-K. thanks the Department of Organic Chemistry, Faculty of Chemistry, Wroclaw University of Technology, Wroclaw, Poland for giving her leave to undertake research at the ANU.

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